Process and device for curing UV printing ink

ABSTRACT

The invention concerns a process and a device for curing a UV curing printing ink ( 14 ) on a printed material ( 9 ), wherein the printing ink ( 14 ) is irradiated with UV light from a UV radiation source ( 8 ). A low pressure gas discharge lamp ( 7 ) is proposed as UV radiation source ( 8 ). The device in accordance with the invention is characterized in that it comprises a stationary reflector ( 5 ) having, in particular, a special reflecting layer with diffuse reflecting material based on silicone rubber having diffuse reflecting particle imbedded therein.

[0001] The invention concerns a process for curing a UV curable printingink on a printed material, wherein the printing ink is irradiated withUV light from a UV radiation source. The invention also concerns anassociated device for irradiating the printing ink with UV light.

[0002] UV curing printing inks contain low amounts of solvent or nosolvent, cure when irradiated, and have recently become increasinglyimportant. This is due to the high energy of UV radiation which isparticularly advantageous in high speed printing of printed materials,in particular for flat bed printing and letter press printing. They alsohave practical advantages from a technical applications point of viewcompared to solvent-containing ink e.g. with regard to their workinglifetime, solvent related environmental pollution, and waste disposal.

[0003] UV curing printing inks have a UV curable fixing agent systemcomprising a polymerizing fixing agent or mixture of fixing agents andone or more associated photo-initiators. The polymerization orcross-linking can be triggered by UV irradiation to cure the ink. Onedifferentiates between radical-induced and cationic polymerization.Conventional radical-induced polymerizing fixing agents are based onacrylates, whereas the cationic polymerizing ones are characterized byacid release during UV irradiation. The invention concerns the generalcuring of UV curing printing inks independent of the particular fixingagent system.

[0004] Conventional applications for UV curing printing inks are e.g.:sheet-fed offset printing (e.g. packaging), continuous offset printing(e.g. direct mail advertising), dry offset printing (indirectletterpress printing, e.g. cups and tubes), label printing (letterpressand flexographic printing), flexographic printing (e.g. packaging) andsilk screen printing (e.g. technical articles). UV curing, also oftenreferred to as UV drying, has the advantage that the printing inks aresolvent-free or of low solvent content and cure rapidly on the printedmaterial under UV irradiation so that the printed material can bepromptly further processed or packaged. The invention concerns curing ofthe printed ink and is therefore independent of the particular printingprocess used for introducing the printing ink onto the printed material.

[0005] Substantial technical requirements are required for theindustrial radiation curing of printing ink. Prior art has required veryhigh output power for the UV radiation sources in order to satisfydemands for ever increasing production speeds of 100 to 400 m/min andhigher. In multi-color printing, the separation between printing devicesmust be kept small to guarantee the precise matching of sequentialcolors without excessive complication and expense. The maximumseparations in combination with the high printing speeds lead toextremely short times within which the ink must be sufficiently cured toprevent smearing during subsequent handling. Practical separationsbetween printing devices assume values of circa 0.3 to 1.0 mcorresponding to production times between printing stations of about 0.1sec.

[0006] When one considers these stringent requirements it becomes clearthat the UV intensity of the radiation source is very important. Inorder to achieve this, mercury vapor high pressure and medium pressurelamps have been nearly exclusively used as UV radiation sources inpractical industrial applications up to this point in time. These lampsfacilitate a particularly high UV intensity. DE-3902643 C2 and DE4301718 A1 provide examples therefor.

[0007] The arc lengths of the conventional lamps vary between 10 cm and220 cm and the specific electric power lies in the range between 30 to250 watts per centimeter of arc length. The UV light power assumesvalues of approximately 20 watts per centimeter of arc length. Due tothe need for UV transparency, the tubular lamp material is quartz andthe lamps are operated with a gas pressure of 1 to 2 atm. In certaincases, lasers, in particular excimer lasers, are also used to producethe UV radiation.

[0008] The above mentioned conventional UV radiation sources have theadvantage of being able to produce a very high UV intensity on thesurface of the printed material to effect very short curing times in therange of tenths of seconds. Excimer lasers have the disadvantage ofbeing complicated and expensive. For this reason, medium and highpressure gas discharge lamps are more widely used. They have, however,the disadvantage that their efficiency of UV light production in therelevant spectral region is only 20%, so that 80% of the introducedenergy is dissipative power and must be removed by cooling.

[0009] Due to the high power consumption and high dissipative power ofthe lamps, their surface temperatures are in the range of 800 to 900° C.which necessitates special measures for cooling their surroundings.Since the lamps cannot be immediately restarted after having beenswitched-off, one must also provide means for preventing the printingink introduced onto the printed material or the printed material itselffrom burning when the printing machine is in paused operation. Heatprotection glass, which is sometimes cooled, and pivoting reflectors aretherefore provided. The power consumption portion of the drying deviceused in a conventional printing machine having an overall powerconsumption of 100 kW, assumes values in excess of 50 kW and typically80 kW.

[0010] Conventional use of medium pressure and high pressure lamps istherefore very complicated and expensive and is associated with highpower consumption. The associated disadvantages have, however, beenaccepted in the art of radiation curing printing technology, since onehas assumed up to this point in time that very high intensity UV lampshaving high UV radiation power were necessary to achieve shorter curingtimes.

[0011] The publication Industrie-Lackier-Betrieb [Industrial Coating andPainting], 1969, pages 85-91 proposes the use of so-called actinic orsuper-actinic fluorescent lamps to reduce thermal loads when curing UVcurable coatings. These are special low pressure lamps having afluorescent coating which shifts the intensity maximum towards the redto achieve a spectrum having high fractions in the UV-A region. The highUV-A fractions have been considered necessary by those of average skillin the art in order to achieve rapid reaction times. Those skilled inthe art of curing pigmented systems such as printing inks were of thesame opinion.

[0012] JP 59189340 A2 (Derwent reference No. 84-303796/49) proposes acompound for use as printing ink which can be cured by a plurality ofdifferent UV radiation sources, including high pressure, mediumpressure, and low pressure mercury lamps. The applications described inthis publication suggest that the lamps primarily emit in the UV-A orvisible spectral region and that relatively long irradiation times, notcompatible with rapid industrial production processes, were required.

[0013] Departing from this prior art, it is the underlying purpose ofthe invention to create a process and an associated device for curing aUV curable printing ink on a printed material which avoids thedisadvantages of conventional UV gas discharge lamps associated withtheir high heat production.

[0014] In order to achieve this purpose, it is proposed in the abovementioned process and corresponding device to use a low pressure gasdischarge lamp as the UV radiation source which has an integratedspectral radiation flux in the UV-B and UV-C region in excess of 50%,preferentially in excess of 75% of the UV radiation flux.

[0015] In accordance with the present invention, one has surprisinglydiscovered that the extremely stringent requirements for radiationcuring of printing inks can be satisfied by low pressure gas dischargelamps without—as had been previously considered necessary—having theirwavelength spectrum somewhat shifted or substantially shifted towardslonger wavelengths.

[0016] The range of the UV spectrum as well as its subdivision intovarious regions are not consistently defined in the literature. Withinthe framework of the invention, the UV spectrum is subdivided inaccordance with DIN 5031, Part 7. It includes the region between 100 and380 nm, wherein the UV-C range extends from 100 to 280 nm, the UV-Bregion from 280 to 315 nm and the UV-A region ranges from 315 to 380 nm.Spectral radiation flux refers to the radiation power in watts per nm asa function of wavelength. The radiation flux is a measure of theintensity of the radiation. Integration or summation of the spectralradiation flux over a wavelength interval gives the radiation fluxirradiated within this wavelength interval.

[0017] Low pressure gas discharge lamps in accordance with the inventionare lamps which can normally be operated with gas pressures between 10mbar and 50 mbar, preferentially between 20 mbar and 30 mbar. Theirspecific electric power consumption is substantially less than that ofmedium and high pressure lamps and lies in the range of 0.2 to 2.5,preferentially between 0.5 and 1.0 watts per cm of arc length. Althoughthe low pressure gas discharge lamp efficiency for the relevant UVregion is higher than that of conventional lamps and amounts to 30 to40%, their overall UV radiation flux is substantially less than that ofconventional lamps. It assumes values of circa 0.2 watts per centimeterof arc length and is therefore about a factor of 100 less than that ofpreviously used conventional medium and high pressure lamps.

[0018] It has surprisingly turned out that UV curing printing inks canalso be satisfactorily cured using low pressure gas discharge lamps evenwhen the printing ink is irradiated with a UV intensity of radiationbetween 1 and 100 mW/cm², preferentially between 10 and 50 mW/cm². TheUV intensity of radiation of medium and high pressure lamps on theprinted material is approximately 1 W/cm². This radiation intensity onthe printed material refers to the radiation flux per unit area incidenton the printed material. The printed material can be tilted at an anglewith respect to the direction of the radiation. The intensity ofradiation has the units of W/cm².

[0019] The use of low pressure gas discharge lamps in accordance withthe invention has various significant advantages in practicalapplications. Their surface temperature is substantially lower. Mercuryvapor lamps have temperatures during normal and optimized operation ofabout 30° C. Amalgam lamps, which have the advantage compared to mercurydischarge lamps of having a somewhat higher UV light yield, havetemperatures during normal operation of circa 120° C. These lowersurface temperatures, in combination with the reduced power consumption,lead to a substantially reduced temperature loading of the surroundingsof the lamp and of the printed material.

[0020] The reduced heating of the counter-pressure cylinder also hastechnical advantages, in particular for multi-color printing devices. Upto this point in time, a high degree of complication and expense wasnecessary to keep the counter-pressure cylinder at a constanttemperature. This was of central importance for the quality andexecution of the printing process. The reduced temperature loadingallows for the printing of materials with UV curable printing ink whichcould not previously be printed. An example is temperature-sensitiveplastic foils (e.g. heat shrinking foils).

[0021] The relatively low power requirements of low pressure gasdischarge lamps and the technical simplicity of associated coolingallows for the reduction of the fraction of power consumption for thedrying unit: in a printing machine having an overall power consumptionof 100 kW, to about 10 to 15 kW or less. The power consumption of amedium pressure gas discharge lamp with associated cooling fan assumes atypical value of about 3.5 kW. In contrast thereto, the powerconsumption of 10 low pressure gas discharge lamps in accordance withthe invention, including associated fans, is only approximately 400 W.

[0022] In addition to the reduced heat load, the reduced danger ofburning, and the reduced power loss, the low pressure gas dischargelamps have the added advantage of being quickly exchangeable. The lampsrequire nearly no cooling-down time following failure and can thereforebe more rapidly replaced. Furthermore, low pressure gas discharge lampshave the additional advantage compared to conventional lamps ofrequiring little or no warm-up time before reaching stable operatingconditions. They can also be restarted immediately after beingswitched-off and have an intensity which can be regulated. In addition,in contrast to medium pressure lamps, there is no danger that drops ofink or dirt particles burn into the bulb to destroy the lamp. Thelifetime of low pressure gas discharge lamps is about 8000 hours, whichis at least four times that of medium pressure lamps.

[0023] In addition, the amount of ozone produced through operation oflow pressure gas discharge lamps is substantially less than that ofmedium pressure lamps. This is due to the fact that low pressure gasdischarge lamps emit little or no radiation at the critical wavelengthof 185 nm at which ozone is produced in atmospheric oxygen. In contrastthereto, medium pressure lamps cause substantial ozone production.

[0024] In summary, the invention achieves goals which have been soughtby those of average skill in the art for a long time. The followingmeasures are preferentially used individually or in combination toassure particularly good results with regard to quality and speed of theconfiguration as well as with regard to the structural requirements ofthe printing machine.

[0025] It can be advantageous when the integrated UV-B spectralradiation flux is in excess of 50%, preferentially more than 75% of theUV radiation flux. In this case, the lamps are designated UV-B lamps. Itcan also be advantageous if the UV-C integrated spectral radiation fluxis in excess of 50%, preferentially more than 75% of the UV radiationflux. In this case, the lamps are designated UV-C lamps.

[0026] Within the framework of the invention, both UV-C as well as UV-Blow pressure gas discharge lamps have turned out to be advantageous forthe curing process. With UV-C low pressure gas discharge lamps, theradiation flux integrated over the UV-C range can assume values inexcess of 50%, preferentially more than 75% of the UV radiation flux. Inthe case of a UV-B low pressure gas discharge lamp, the correspondingUV-B integrated spectral radiation flux can assume values in excess of50%, preferentially more than 75% of the UV radiation flux.

[0027] The maximum of the spectral radiation flux distribution of thelow pressure gas discharge lamps, in particular of the UV radiationflux, can advantageously lie in the UV-B or UV-C region. For a linespectrum, this refers to the wavelength having the highest UV intensity.For a continuous spectrum, this requirement refers to the maximum of thespectral radiation distribution. When the UV spectrum has both lines andcontinua, this feature refers to the maximum with regard to the line andcontinuous emission regions.

[0028] An additional advantageous feature proposes use of a low pressuregas discharge lamp having an integral spectral UV radiation flux above awavelength of 190 nm, in particular above 240 nm, which is more than50%, preferentially more than 75% of its UV radiation flux, inparticular of its UV-C radiation flux. It is particularly advantageouswhen the integrated UV-C radiation flux above a wavelength of 190 nm, inparticular above 240 nm, is more than 50% and preferentially more than75% of the UV radiation flux.

[0029] It is particularly advantageous when the low pressure gasdischarge lamp emits more than 50%, preferentially more than 75% of theradiation flux of its UV light in the UVC region above a wavelength of240 nm. The lamp then differs substantially from the medium pressurelamps having their main emitted UV spectral fraction in the UVB or UV-Aregion. Since not only the overall intensity but also the distributionof the individual lines can be of significance, it is advantageous whenthe above mentioned conditions apply to wavelengths having an intensityof more than 20% of the UV wavelength of highest intensity. Theintensity maximum of UV-C low pressure gas discharge lamps is normallyin the wavelength range between 249 and 259 nm, in particular at 254 nm.

[0030] The likewise advantageous UV-B low pressure gas discharge lampsare also designated as UB-B fluorescent lamps. They have a phosphorcoating which shifts the maximum of the radiation flux into the UV-Bregion. The maximum preferentially lies above 305 nm. The actualpositions of the intensity maximum and of the emitted lines as well as,in particular, their linewidths can be influenced by the phosphor or thephosphor mixture. The possible band widths thereby range from verynarrow, nearly monochromatic UV-B radiation up to emissions which nearlyspan the entire UV-B range. The UV-B low pressure gas discharge lampsadvantageously emit in excess of 50%, preferentially more than 75% oftheir UV light in the UV-B region.

[0031] Within the framework of the invention, low pressure gas dischargelamps are generally preferred whose emission spectra are not shiftedtowards longer wavelengths through the addition of fluorescentmaterials. This means that neither an actinic nor a super-actinic gasdischarge lamp is used. The UV-B lamps are not quite as advantageous asthe UV-C lamps, since their light output is lower due to the lightconversion step and since the printing ink can be less reactive in thespectral region in which they emit than in the UV-C region. Theynevertheless likewise constitute an economically interesting improvementover conventional medium pressure and high pressure lamps.

[0032] In accordance with an additional advantageous feature, aplurality of low pressure gas discharge lamps of differing emissionspectra can be utilized, in particular a combination of a UV-C and aUV-B low pressure gas discharge lamp.

[0033] The advantageous use of low pressure gas discharge lamps havingdiffering emission spectra to produce mixed light can be effected notonly by having differing lamps, but also using lamps which are partiallycoated with fluorescent material. The ratio of integrated UV-B tointegrated UV-C radiation flux can lie between 0:1 and 1:0. A higherUV-C fraction is however normally preferred for the above mentionedreasons.

[0034] The fixing agents of conventional UV cured printing inks arenormally tuned to the particular radiation of the UV radiation source.One would therefore expect that conventional printing inks are notsuitable for the invention and that special fixing agent systems or, inparticular, special photo-initiators would be necessary which are tunedto the UV spectrum of the low pressure gas discharge lamps used inaccordance with the invention. It is clearly possible for one of averageskill in the art to develop printing ink compositions andphoto-initiators which are optimized and specially tuned to low pressuregas discharge lamps. It has however surprisingly been discovered withinthe framework of the invention that good curing results can also beachieved using conventional printing inks. This is particularly true fore.g. the XKC-Series UV-Flex inks of Gebruder Schmidt Druckfarben,Frankfurt, in particular of type 80 XKC 1004-1.

[0035] A printing ink is, by way of example, suitable for the process inaccordance with the invention which has a fixing agent system containingthe following components: a) one or a plurality of cycloaliphatic epoxyresins as curable fixing agent and b) one or a plurality ofarylsulfonium salts as photo-initiators. A cycloaliphatic epoxy resin isa cationic curable fixing agent. Clearly, the ink can also containadditional conventional components such as additional photo-initiators,solvents, pigments, dyes, thinning agents, reactive thinner, wax,leveling agent, wetting agents or other additives.

[0036] In accordance with an additional advantageous feature, componentb) contains a triarylsulfonium salt. It is thereby preferred when thetriarylsulfonium salt contains a triarylsulfoniumantimonate, inparticular a triarylsulfoniumhexaflouroantimonate. One canadvantageously further provide that the component b) contains a mixtureof different arylsulfonium salts. The printing ink can also containother fixing agents in addition to the cycloaliphatic epoxy resin.

[0037] Printing technology primarily uses radical curable printing inks,since these provide, when irradiated with conventional medium pressurelamps, a drying time which is shorter than that of a cationic curableprinting ink. The radical curable inks have the additional advantagethat their chemical composition can be widely varied. However, the mostprevalent fixing agents absorb mostly in the UV-C range. As a result,only a small amount of printing ink reactivity can be effected even withphoto-initiators absorbing in the UV-C region. In contrast thereto, thefixing agents used with cationic curable printing inks are substantiallytransparent in the UV-C region. A high degree of reactivity cantherefore be achieved even with a UV-C or UV-B low pressure gasdischarge lamp. Cationic curable inks based on epoxy resins arepreferred within the context of the invention for the above mentionedreasons. Radical curable inks can however also be used.

[0038] It is generally advantageous to cure a printing ink with theprinting process in accordance with the invention which has fixing agentcomponents which are substantially transparent in the UV-C or UV-Bregions of the UV-light from the low pressure gas discharge lamp. Inthis manner, deeper layers can also be reached by adequate amounts of UVlight. This means that the absorption curve of the fixing agent shouldbe shifted towards shorter wavelengths compared to standard fixingagents used in medium and high pressure lamps. Offset printing isassociated with typical layer thickness of 1 to 3 μm and flexographicprinting with layer thickness between 3 and 8 μm. In addition, thesqueezed edges have a thickness of at most 20 μm. The fixing agentshould therefore be sufficiently transparent up to a thickness of 20 μm.This implies that the transparency of the fixing agent is preferentiallysufficiently high up to this layer thickness that it does not absorbmore than half of the incident UV intensity of the low pressure gasdischarge lamp. Accordingly the system of fixing agent and photoinitiator is such that preferentially more than 10% of the UV light isabsorbed up to a layer thickness of 20 μm.

[0039] The properties of the fixing agent, in particular itstransparency to the UV light used and the reactivity of the fixing agentand photo-initiator system are of particular importance for the use of aprinting ink within the context of the invention. In addition, as isusual, the individual components should be mixable and mutuallycompatible so that no spontaneous reactions are triggered. Fillingagents and additives can be used in liquid or solid form and are subjectto the same requirements with regard to UV light transparency as is thefixing agent.

[0040] The pigments can be organic or inorganic in nature. Inorganiccompounds are generally solids and organic compounds can be solid orliquid. The concentrations and absorption properties of liquid pigmentsshould be appropriately adjusted. This is also true for solid pigmentswith which additional grain-size dependent scattering effects can occur.

[0041] The printing ink should be sufficiently reactive to the UV lightand capable of activation by same. This is particularly true for thephoto-initiators which should be sufficiently reactive in the wavelengthrange used. The reactivity has two aspects. First of all, the UV lightabsorption must be sufficiently large. In addition, the photo-initiatorsshould also properly transfer or feed the absorbed energy into thecorresponding radicals (radical polymerization) or acids (cationicpolymerization) to initiate the chain reaction for polymerization. Thephoto-initiator should therefore be present in suitable concentrationsand be sufficiently absorbing. It must also be capable of transferringthe absorbed UV light energy to the monomers for both radical as well ascationic curing.

[0042] It is also possible to use a plurality of photo-initiators in oneprinting ink having different absorption properties. The chain reactioninitiators then differ from the light absorption activators.

[0043] It is generally advantageous to cure a printing ink having fixingagents largely transparent to the UV light emitted by the low pressuregas discharge lamp with photo-initiator components which highly absorbthe UV light emitted by the low pressure gas discharge lamp, which arereactive and which can also be activated in this wavelength region. Itis therefore generally advantageous for the fixing agent andphoto-initiator components of the printing ink to be composed andadapted to each other in such a manner that the printing ink can becured up to a layer thickness of 20 μm using the UV light emitted by thelow pressure gas discharge lamp.

[0044] It is furthermore preferred when the printing ink has a highreactivity even at room temperature. In the process in accordance withthe invention, the printing ink is heated only slightly or not at all.The temperature during UV curing is preferentially not higher than 40°C. Conventional high and medium pressure lamps have substantially highertemperatures and associated application related disadvantages.

[0045] In accordance with an advantageous feature, the time duration ofthe UV irradiation curing of the printing ink is less than 2 seconds,preferentially less than 1 second. The short printing ink reaction timeadvantageously allows for the realization of high production speeds orsmaller separations between the individual printing stations. Thereaction time is thereby that time which passes until the surface of theprinting ink is no longer sticky so that the printed material can beprinted in additional printing stations or otherwise processed. Thecuring time can be substantially longer. With radical curing printinginks, the curing time is not substantially longer than the reactiontime. For cationic curing printing inks, the UV irradiation normallyonly initiates the process, i.e. pre-cures. Subsequent complete curingcan be rapid or could also take up to 24 hours. As mentioned, the shortirradiation time or reaction time is not only of significance for theprinting of an increased number of objects per unit time, rather also inmulti-color printing. The passer problem associated therewithnecessitates small separations between printing stations and associatedrapid intermediate drying to prevent spreading of ink.

[0046] The amount of time during which the printing ink is irradiatedwith UV light depends on the speed with which the printed material andassociated printing ink move relative to the low pressure gas dischargelamps for UV curing as well as on the area irradiated by the lowpressure gas discharge lamps. With the method in accordance with theinvention, printing processes can be advantageously carried out withwhich the printed material moves with a path velocity of more than 20m/min., of preferentially more than 40 m/min., and particularlypreferably of more than 50 m/min.

[0047] The use of low pressure gas discharge lamps in accordance withthe invention, in particular in combination with the above described UVcuring printing inks, has turned out to be particularly advantageous inflexographic printing. This is true for all flexographic printingmachine concepts which can be classified as follows:

[0048] 1. The multi-cylinder printing machine is a rotating type havingfour or six individual printing devices associated with one station, inparticular for printing a plurality of colors.

[0049] 2. The tandem printing machine is a rotating type with eachprinting device disposed in its own individual station.

[0050] 3. The one cylinder or central cylinder machine is a rotatingtype having the printing device disposed about one common centralcounter-pressure cylinder.

[0051] The device in accordance with the invention for curing a UVcuring printing ink on a printed material by means of which the printingink is irradiated with UV light from a UV light source, in particularfor carrying out a process in accordance with the invention, ischaracterized in that the UV radiation source comprises a low pressuregas discharge lamp whose integrated UV-B and UV-C spectral radiationflux is in excess of 50%, preferentially more than 75% of the UVradiation flux. A device of this kind is subsequently designated withthe conventional name “drier”.

[0052] Depending on the application, the drier can include one or moreUV radiation sources. If a plurality of UV radiation sources are used,these could be the same or of differing types. It can also beadvantageous in certain special cases to provide for conventionalradiation sources in addition to the low pressure gas discharge lamp.The exclusive use of low pressure gas discharge lamps is preferred. Anadvantageous feature proposes that a drier comprises more than four,preferentially more than eight, low pressure gas discharge lamps.

[0053] An advantageous embodiment can be particularly characterized bythe fact that the drier comprises a plurality of adjacently disposed lowpressure gas discharge lamps. In this fashion, a high UV radiationintensity per unit area on the printed material or a smoother spatialillumination can be effected. Alternatively, a relatively large area canthereby be illuminated. The low pressure gas discharge lamps can bebar-shaped. However, it is preferred when the drier comprises aplurality of U-shaped low pressure gas discharge lamps disposed next toeach other at the longitudinal sides of the U-shape. U-shaped lowpressure gas discharge lamps have the advantage of effecting arelatively high illumination intensity. The low pressure gas dischargelamps can be arranged in particularly close proximity to another if theyare disposed in alternately opposite directions. The open and closedends of the U-shape form an alternating series and the open ends havingelectrical contacts can be connected to electrical contact elementswithout having the separation between the low pressure gas dischargelamps be limited by these contact elements.

[0054] The separation between the lamps and between the lamps and theprinted material is preferentially subject to the requirement that theradiation intensity in the plane of the printed material within theprincipal effective region, i.e. excluding e.g. the entrance and exitzones, is as homogeneous as possible. If the printed object is movedalong a transport direction and/or rotated in the curing zone, thiscondition applies to the time-integrated intensity during passagethrough the drier.

[0055] The low pressure gas discharge lamps can be disposed at closerelative separations to effect a compact assembly and/or to realize ahomogenous irradiation intensity which deviates by less than 30%,preferentially by less than 20% from an average value. The low pressuregas discharge lamp bulbs can even touch without mutual separation. Theseparation between the bulbs of the low pressure gas discharge lampspreferentially does not exceed 30%, preferentially not more than 20% ofthe low pressure gas discharge lamp bulb diameter.

[0056] The separation between the low pressure gas discharge lamps andthe printed object should be sufficiently large to prevent contact withthe lamp in case that the position of the printed object should besubject to variations. A reasonable practical minimal separation is 1cm. The upper limit of the separation between the surfaces of the lowpressure gas discharge lamps and the printed material can advantageouslybe less than 5 cm.

[0057] It is furthermore preferred when the device comprises a reflectorfor reflecting the UV light emitted by the low pressure gas dischargelamps onto the curing printing ink. The reflector can be used to reflectUV light for UV curing which is not emitted by the low pressure gasdischarge lamps in a direction towards the printed material as well asto effect a more even illumination of the printed material. If theprinted materials are extended, the reflector is preferentially disposedon that side of the low pressure gas discharge lamps which faces awayfrom the printed material to reflect the UV light emitted by the lowpressure gas discharge lamps in a direction towards the printedmaterial. If the printed material is not an extended flat object, it canalso be advantageous to dispose a reflector at that side of the printedmaterial facing away from the irradiation source to more evenlyilluminate the printed material at all sides.

[0058] Reflectors disposed at that side of the low pressure gasdischarge lamps facing away from the printed material are also known inthe art for use with medium and high pressure lamps. They normallycomprise metal plates and can be pivoted to reduce the heat load on theprinted material during pauses in the operation of the installation. Areflector in accordance with the invention can, in contrast thereto,preferentially be stationary. The heat loading of the printed materialcaused by the low pressure gas discharge lamps is not severe. Since thelamps can be immediately restarted, they can also be switched-off ifnecessary. The reflector is therefore less complicated and lessexpensive.

[0059] The reflecting layer of the reflector can be assembled fromplanar portions. In a preferred configuration which is particularly easyto manufacture, the reflector comprises one single planar reflectinglayer. If the reflector is also stationary, the configuration isparticularly simple to realize.

[0060] Improved optics can be achieved in other advantageousconfigurations in which the reflecting layer of the reflector hasconcave portions curved with respect to the low pressure gas dischargelamp. In addition, the reflector can be arranged in a conventionalmanner at a separation from the low pressure gas discharge lamp. Thereduced surface temperature of the low pressure gas discharge lamps alsoallows for the reflector to be in line or surface contact with the lowpressure gas discharge lamp. In this manner, a very compact constructionof the drier is effected which nevertheless has increased light output.Surface contact can advantageously occur over 30% to 60% of the surfaceof the bulb or of the periphery of a cross section through the lowpressure gas discharge lamp respectively. The optimal values for eachcase depend on the size of the printed object and its separation fromthe low pressure gas discharge lamp.

[0061] It can be advantageous when the reflector comprises a dielectricmirrored layer to achieve a high degree of reflection. A dielectricmirrored layer is a multi-layered system of optical coatings forincreasing the amount of reflection. The reflector itself can thereby befashioned from metal, glass or another suitable material.

[0062] The reflector is preferentially diffuse reflecting in order toachieve a more even spatial irradiation intensity on the printedmaterial and i.e. comprises a reflecting layer made from opticallydiffuse reflecting material. Optically diffuse reflecting materials arematerials which, due to their composition, diffusely reflect incidentoptical radiation or diffusely pass penetrating radiation. They cantherefore be designated as Lambert surfaces or Lambert radiators. Theyare usually mat white.

[0063] The optically diffuse reflecting material can be made fromconventional ceramic plate or from metallic reflectors having aroughened, metallic reflecting surface (e.g. aluminum plates). A coatingcan be used comprising, in particular, a transparent material containingdiffuse reflecting particles such as barium sulfate, titanium oxide ormagnesium oxide.

[0064] A particularly advantageous feature proposes that the opticallydiffuse reflecting material of the reflector reflecting layer comprisesa matrix made from a transparent matrix material consisting essentiallyof a curable silicone rubber with imbedded reflecting particles. Amaterial of this kind is optically, chemically, biologically, andthermally resistant, is insensitive to soiling and can also be easilycleaned. It has a good resistance to aging and has high transparency, inparticular to UV.

[0065] The matrix material in accordance with the invention consistsessentially of silicone rubber. By essentially is meant that thesilicone rubber does not contain any amount of foreign materials whichwould be intolerable for obtaining the desired properties, so that theproperties of the matrix material are determined by the silicone rubber.As a rule, the matrix material normally consists of silicone rubberhaving a standard commercial or preferentially higher purity of e.g. inexcess of 95%.

[0066] In principle, all conventional silicone rubber is usable withinthe framework of the invention. A suitable silicone rubber material canbe selected which has the necessary matrix material properties independence on the application. Both condensation cross-linked as well asaddition cross-linked rubber may be used.

[0067] Silicone rubbers can be advantageously poured to facilitateinexpensive creation of arbitrary shapes for various applications. Othereconomical production processes, such as extrusion, are advantageous andpossible. The thin liquid curable silicone rubber is initially processedand vulcanization subsequently occurs to form a cured, solid matrixmaterial. For most applications, it is advantageous when the Shore Ahardness of the cured matrix material assumes values in accordance withthe DIN standard 53505 of between 20 and 90. The matrix material has anadvantageous intrinsic solidity in this range.

[0068] In accordance with an advantageous feature, the reflectingparticles are present within the matrix material in powdered form. Formost applications, the reflecting particles should be homogeneouslyimbedded in the matrix material. For special applications, it canhowever be advantageous for the concentration of reflecting particles inthe matrix material to increase or decrease with depth.

[0069] All conventional diffuse reflecting substances are suitable foruse as diffuse reflecting particles in accordance with the invention.Examples of such diffuse reflecting substances are magnesium oxide,aluminum oxide, titanium dioxide, polytetrafluoroethylene (Teflon (^(R))or silicon dioxide (Aerosil (^(R)). Barium sulfate has turned out to beparticularly advantageous within the context of the invention.

[0070] The diffuse reflecting particles substantially include one ormore of the above mentioned substances. By substantially is meant thatother particles are not present in the material or are only present tosuch an extent that, for the actual application, the diffuse reflectionproperties are determined by the particles to satisfy the particularrequirements in each case. The particles are normally contained withinthe matrix material as pure substances having a high commerciallyproducible purity, e.g. in excess of 99%. A high purity and homogeneousdistribution can be particularly advantageous in optics applications.The particles of each substance can have one grain size or comprise amixture of differing grain sizes to achieve special spectral properties.

[0071] The reflecting particles in the material in accordance with theinvention can comprise only one of the above mentioned substances or canbe a mixture of two or more differing substances. For production relatedtechnical reasons, an admixture of particles of only one substance ispreferred. In special applications, in particular for effecting aspecific spectral dependence, it can however be advantageous to utilizea mixture of differing substances and/or a mixture of differing grainsizes.

[0072] The grain size of the above mentioned particles advantageouslylies substantially between 1 μm and 100 μm: for the case of silicondioxide (Aerosil (^(R)), between 10 nm and 200 nm. Substantially therebymeans that the average value of the grain size distribution lies in thisrange. Since the grain size of particles or of powder has a certaintolerance or grain size distribution in dependence on productionprocesses, a small amount e.g. up to 5% of the particles can also bepresent which lie outside of the above mentioned size range.

[0073] The full width half maximum of the actual grain size distributioncan be critical in certain applications and rather insignificant forother applications. Trial and error can determine which particle sizesand particle size distributions produce the desired reflectionproperties for the actual case at hand.

[0074] The material in accordance with the invention has the advantageof having a wide range of applicability. It can be easily manufacturedand tailored mechanically and optically to the case at hand. It can beself-supporting and of nearly arbitrary shape or be securely disposed ona substrate, wherein it can level and cover unevenness in the substrate.It is optically, thermally and biologically stable andtemperature-insensitive. It can be easily cleaned and absorbs littlelight. The actual properties can be optimized for the particularapplication requirements. In addition, it is easily and thereforeinexpensively processed. The hardness can be adjusted within a widerange to facilitate many differing applications. For example, flexiblemats of suitable stability can be produced e.g. by molding which havearbitrary curved and bent shapes. The material can be easily worked andmechanically processed. It can be solid or flexible and can also beglued. Shaped objects can be molded or produced by injection molding.The material has no intrinsic color and therefore does notdisadvantageously influence the spectrum. The surface of the reflectingmaterial facing the emitted light must not have the mat finish requiredwith conventional materials. It must not have a “molecular roughness” inorder to effect good diffuse reflection performance. For this reason, itcan be molded in a mold having a smooth surface.

[0075] The material in accordance with the invention is preferentiallyproduced by mixing of the particles into a liquid matrix material undervacuum. In this manner, a vulcanized material can be produced withoutbubbles.

[0076] Further advantageous features and highlights can be recognized bymeans of the following embodiments of the invention and are described infurther detail below with reference to the schematic representation ofthe drawings.

[0077]FIG. 1 shows a schematic cross section through a drier accordingto prior art in an operating state,

[0078]FIG. 2 shows a schematic cross section through a drier accordingto prior art in a paused condition,

[0079]FIG. 3 shows a modification of FIG. 1,

[0080]FIG. 4 shows a modification of FIG. 2,

[0081]FIG. 5 shows a schematic cross section through a first drier inaccordance with the invention,

[0082]FIG. 6 shows a first modification of FIG. 5,

[0083]FIG. 7 shows a second modification of FIG. 5,

[0084]FIG. 8 shows a perspective view of FIG. 5,

[0085]FIG. 9 shows a perspective view of FIG. 6,

[0086]FIG. 10 shows a perspective view of FIG. 7,

[0087]FIG. 11 shows a modification of FIG. 8,

[0088]FIG. 12 shows a modification of FIG. 9,

[0089]FIG. 13 shows a modification of FIG. 10,

[0090]FIG. 14 shows a schematic view of a plurality of lamps,

[0091]FIG. 15 shows a modification of FIG. 14,

[0092]FIG. 16 shows a schematic cross section through a drier and aprinting machine,

[0093]FIG. 17 shows a schematic cross section through a drier inaccordance with the invention,

[0094]FIG. 18 shows a detail of FIG. 17,

[0095]FIG. 19 shows a relative spectral radiation flux of a highpressure mercury vapor lamp,

[0096]FIG. 20 shows a relative spectral radiation flux of a low pressuregas discharge mercury vapor lamp, and

[0097]FIG. 21 shows a spectral radiation flux of a UV-B low pressure gasdischarge lamp.

[0098]FIG. 1 shows a schematic cross section through a drier 20according to prior art in its operating state, with curing printedmaterial 9 which has been printed with UV curing printing ink 14 passingthereby. A medium pressure gas discharge lamp, UV radiation source 8,produces UV light to trigger polymerization of the printing ink 14. Theprinted material 9 is fed past the UV radiation source 8 in transportdirection 10. Pivoting reflectors 21 are provided for smoothing out theillumination intensity on the printing ink 14 and for increasing thelight yield. They can each be pivoted via a turning device 11 from theoperating condition shown in FIG. 1 into the paused position shown inFIG. 2. The reflectors 21 must be pivoted, since the medium pressurelamp has a very high surface temperature and would burn the printedmaterial 9 when it is stationary relative to the lamp 8.

[0099] Heat protection glass 22 is disposed between the UV radiationsource 8 and the printed material 9 to protect the printed material 9and the printing ink 14 from the high amount of heat emanating from themedium pressure lamp. FIGS. 3 and 4 are modifications of FIGS. 1 and 2having cooling pipes 35 flown through by water instead of the heatprotection glass 22 to remove the heat.

[0100]FIG. 5 shows a schematic cross section through a drier 20 inaccordance with the invention. In this case, the UV radiation sources 8comprise a plurality of mutually adjacent low pressure gas dischargelamps 7 which are passed by the printed material 9, printed withprinting ink 14, which is fed in transport direction 10 to cure theprinting ink 14. Advantageous low pressure gas discharge lamps are, inparticular, the UV-C lamps of type TUV produced by Philips having aprincipal emission at 254 nm and the UV-B lamps of type TL/01 withprincipal emission at 311 to 312 nm or of type TL/12 having principalemission at 306 nm. They have a high efficiency for UV light and can beoperated with nearly no ozone production. The low heat output of the lowpressure gas discharge lamps permits the reflector 5 to be stationaryand at a small separation from the lamps. The separation between thereflector 5 and the UV radiation sources 8 can be less than twice thediameter, preferentially less than one time the diameter, of the bulb 16of the UV radiation sources 8.

[0101] In the embodiment shown, the reflector 5 comprises three planarreflectors 5. One large reflector 5 a is disposed on that side of thelamps 8 facing away from the printed material 9. Two smaller reflectors5 b are disposed at the sides. The reflectors comprise a reflectinglayer made from a reflecting material 1. The reflecting material 1 cane.g. be a conventional ceramic plate or metallic reflector. The metallicreflector can have a roughened, metallic reflecting surface and be madee.g. from aluminum.

[0102] It is preferred when the reflecting layer of the reflector 5consists essentially of an optically diffuse reflecting material inaccordance with the invention having a matrix material made from curedsilicone rubber with a homogeneous distribution of particles imbeddedtherein.

[0103] These particles comprise powdered barium sulfate having a grainsize of about 50 μm. The particles are not visible in FIG. 5 due totheir small size. The ratio of particles to matrix material isapproximately 1:10 by weight. A ratio smaller than 1:100 does notnormally result in sufficiently high reflectivity. Weight ratios inexcess of 1:1 normally result in a degree of filling of the matrixmaterial by the particles which is so high that the silicone becomesbrittle or does not properly vulcanize.

[0104] The reflector 5 has a reflectivity in excess of 90%. Thereflecting layer of material 1 has a thickness of several millimeters.It may advantageously lie in the range between 0.1 and 10 mm. Thereflector 5 can therefore be a so-called volume reflector. Such areflector differs from a purely surface reflector in that reflectionalso occurs from deeper material layers.

[0105] The reflecting material 1 or the reflecting sheet metal isdisposed on a substrate 6 or on a portion of the housing 27. The matrixmaterial 2 or the reflecting material 1 can be a condensationcross-linked silicone rubber directly bound to the supporting surfaceand can, for example, be extruded onto the substrate 6. If the matrixmaterial is an additive cross-linked rubber, a suitable bonding process,e.g. gluing, can be used to bind it to the supporting surface.

[0106] Advantageous silicone rubbers are in particular those sold by thecompany Wacker-Chemie GmbH [Wacker Chemical Incorporated], Munich underthe name “Elastosil (R)”, in particular the types M 4600, R 401, R 402,R 411, R 420, R 4000, and R 4105 as well as Semicosil, in particular thetypes 911, 912, and types RTV-E 604, RTV-ME 601, and SilGel 612.

[0107]FIG. 6 shows a modified reflector 5. The reflecting material 1layer consists essentially of a matrix material in accordance with theinvention made from silicone rubber having diffuse reflecting particles.It is mounted to a substrate 6 or to a housing portion 27.

[0108] The reflecting layer is characterized in that its surface facingthe radiation sources 8 has curved concave portions with respect to theradiation sources. They are disposed at a small separation from thesurface of the bulb 16 of the corresponding UV radiation source 8. Thisseparation can be less than one half of the diameter of the UV radiationsource 8 bulb 16. Towards this end, the center of each curve of thereflector 5 can lie inside the associated low pressure gas dischargelamp, in particular at its center. In this manner a compactconfiguration can be realized which provides for a homogeneousillumination of the printed object 9. For certain applications, it canbe advantageous if the reflecting layer of diffuse reflecting material 1is immediately adjacent to the bulb 16 of the UV radiation sources 8.This is possible, in particular, with low pressure mercury gas dischargelamps.

[0109]FIG. 7 shows a schematic cross section through a drier 20. Thisdrier 20 differs from the driers in accordance with FIGS. 5 and 6 inthat the reflector 5 consists essentially of one or more sheet metalreflectors which are not flat but curved in a concave manner withrespect to the UV radiation sources 8. The reflectors 5 can also bestationary and -an be disposed at a small separation from the lowpressure gas discharge lamps due to their low heat production.

[0110] Perspective views are shown in FIGS. 8 through 10. FIG. 8corresponds to FIG. 5, FIG. 9 to FIG. 6, and FIG. 10 to FIG. 7. In allthe figures, construction elements such as electrical leads, coolingdevices and mechanical supports are not shown for reasons of clarity.

[0111]FIGS. 11 through 13 show modifications of FIGS. 8 through 10 whichdiffer with regard to the transport direction 10 of the printed material9. In FIGS. 8 through 10, the printed material 9 is transported at rightangles to the axial direction of the UV radiation sources 8. With thedriers 20 in accordance with FIGS. 11 through 13, transport occurs inthe axial direction of the UV radiation sources 8. In principle, thetransport direction 10 can assume any arbitrary angle with respect tothe axes of the low pressure gas discharge lamps. The transportdirections 10 shown are preferred for optimizing use of the emitted UVlight and to achieve an illumination time which is evenly distributed onthe printed material 9.

[0112]FIGS. 14 and 15 show schematic views of a plurality of UVradiation sources 8 configured as U-shaped low pressure gas dischargelamps 7. In the embodiment shown, a total of nine lamps are disposedwith mutually adjacent lengthwise sides for even illumination of thedrying surface of the printed material 9. In addition, the lamps arealternately oppositely directed to effect as compact an assembly aspossible with high illumination intensity. The electrical connectionelements 13 therefore form an alternating series with the closed ends ofthe U-shaped lamps at both sides of the arrangement. There is asufficient amount of space between each of the electrical connectionelements 13 such that the separation between the lamps is not limited bythe electrical connection elements 13. FIGS. 14 and 15 differ withregard to the transport direction 10 of the printed material 9 whosedrying surface, having the UV printing ink 14 which is to be cured,passes by the lamps. The reflectors are not shown in FIGS. 14 and 15.

[0113]FIG. 16 shows a schematic cross section through a drier 20 and aprinting machine. Its UV radiation source 8 comprises a conventionalhigh pressure gas discharge lamp emitting in the UV region. In additionto the lamps, the housing 27 also contains pivoting reflectors 21 fordirecting the light onto the printed material 9. When the installationcomes to rest, these reflectors 21 can be pivoted to protect the printedmaterial 9 from overheating. A heat protection glass 22 is also providedfor, since the conventional UV illumination sources 8 generate a largeamount of heat. In accordance with the invention, one or more lowpressure gas discharge lamps should therefore be used as UV radiationsource 8. The pivoting reflector 21 can then be a stationary reflectorin the

[0114] manner mentioned above and the heat protection glass 22 can beeliminated. In this manner, the drier 20 can be of compact constructionand evenly illuminate the printed material 9. The heat load is alsosubstantially reduced.

[0115] In the example shown, the printed material 9 are tubes or cupsdisposed on rotating tube arbors 26 of a tube plate 25. The UV curingprinting ink 14 is introduced from a wiping blade chamber or a colorchamber (not shown) to the printing apparatus with its associated rasterroller 23 and block roller 24. The block roller 24 transfers the patternonto the tubes. The rotating tube plate 25 guides the tubes through thecuring zone of the drier 20 to effect curing via UV irradiation. Afterleaving the curing zone, the tubes are removed from the tube arbors 26and the tube arbors 26 are provided with fresh non-printed tubes. Theassociated mounting and removal devices are not shown.

[0116] In order to achieve a homogeneous illumination of the printedmaterial 9 within the curing zone as well as a high light yield, thecuring zone is surrounded with optically diffuse reflecting material 1in accordance with the invention. The reflecting material 1 can beintroduced onto special substrates 6 or disposed on the housing 27. Theevenness of the illumination and the light yield can be improved, inparticular, by use of reflectors 5 disposed on the side of the printedmaterial 9 facing away from the illumination source 8. If the heatproduced by the illumination source 8 is not excessively high, thepivoting reflectors 21 can also be provided with material 1 inaccordance with the invention. A stationary reflector in accordance withthe invention can alternatively be disposed on the side of theillumination source 8 facing away from the printed material 8.

[0117]FIG. 17 shows the drier of FIG. 16 in an embodiment in accordancewith the invention having low pressure gas discharge lamps 7. Theprinted material 9 are tubes or cups disposed on rotating tube arbors 26of a tube plate 25. They are transported through the drier 20 at a pathspeed of circa 50 m/min. In addition to this motion along a path, thetube arbors 26 also rotate. The drier 20 comprises a housing 27 in whichthe reflecting material 1 is disposed on substrates 6 for effecting ahomogeneous illumination of the printed material in the curing zone. Thereflector 5 provides for homogeneous illumination in combination withthe 12 low pressure gas discharge lamps 7. The low pressure gasdischarge lamps 7 are disposed at close proximity to another and theprinted material 9 is fed past and in close proximity to the lowpressure gas discharge lamps 7. Neither an expensive and difficultcooling mechanism nor a heat protection glass are needed due to the lowheat production of the low pressure gas discharge lamps 7. The reflector5 is stationary and does not comprise any pivoting components. FIG. 18shows a detail of FIG. 17.

[0118]FIGS. 19, 20, and 21 show typical relative spectral radiationfluxes of mercury gas lamps. FIGS. 19 and 20 each show the spectralradiation flux E in arbitrary units as a function of wavelength w and,in FIG. 21, in absolute units as a function of wavelength w. FIG. 19shows the spectrum of a high pressure lamp and FIG. 20 that of a lowpressure UV-C lamp. It can be seen that, the UV-C low pressure gasdischarge lamp emits primarily in the UV-C region, whereas the mainemission region of the high pressure lamp is at longer wavelengths.

[0119] The UV-C low pressure gas discharge lamp of FIG. 20 is a lowpressure lamp which does not have any added fluorescent material, e.g.it is a non-actinic low pressure lamp. FIG. 21 shows the spectrum of aUB-B low pressure gas discharge lamp. This is a fluorescent materiallamp whose principal emissions are displaced into the region near 305 nmthrough the addition of fluorescent material. Further intensities alsoappear in the UV-A and visible region.

1. Process for curing a UV curing printing ink (14) on a printedmaterial (9), wherein the printing ink (14) is irradiated with UV lightfrom a UV radiation source (8), characterized in that, a low pressuregas discharge lamp (7) is utilized as the UV radiation source (8) whichhas a spectral radiation flux integrated over the UV-B and UB-C regionof more that 50% of the UV radiation flux, preferentially more than 75%of the UV radiation flux.
 2. The process of claim 1 , characterized inthat the maximum of the spectral radiation distribution of the lowpressure gas discharge lamp (7) lies in the V-B or UV-C region.
 3. Theprocess of claim 1 or 2 , characterized in that the integrated spectralUV radiation flux, in particular the UV-C radiation flux, of the lowpressure gas discharge lamp (7) above a wavelength of 190 nm, inparticular above 240 nm, is in excess of 50%, preferentially more that75%, of its UV radiation flux, in particular of its UV-C radiation flux.4. The process of one of the preceding claims, characterized in that theintegrated UV radiation intensity, in particular the integrated UV-B andUV-C radiation intensity or, in particular, the UV-C radiationintensity, is between 1 and 100 mW/cm², preferentially between 10 and 50mW/cm².
 5. The process of one of the preceding claims, characterized inthat the printing ink (14) is not heated above 40° C. during the UVcuring.
 6. The process of one of the preceding claims, characterized inthat the printing ink (14) has high reactivity at room temperature. 7.The process of one of the preceding claims, characterized in that thetime duration of irradiation for curing the printing ink (14) is lessthan two seconds, preferentially less than one second.
 8. The process ofone of the claims 1 through 7, characterized in that the printing ink(14) comprises the following components: a) one or more cycloaliphaticepoxy resins as curable fixing agent, and b) one or more arylsulfoniumsalts as photo-initiator.
 9. The process of claim 8 , characterized inthat component b) contains a mixture of differing arylsulfonium salts.10. Device for curing a UV curing printing ink (14) on a printedmaterial (9), by means of which the printing ink (14) is irradiated withUV light from a UV radiation source (8), in particular for carrying outthe process according to one of the claims 1 through 9, characterized inthat the UV radiation source (8) comprises a low pressure gas dischargelamp (7) having a spectral radiation flux integrated over the UV-B andUB-C region of more that 50% of the UV radiation flux, preferentiallymore than 75% of the UV radiation flux.
 11. The device of claim 10 ,characterized in that the maximum of the spectral radiation distributionof the low pressure gas discharge lamp (7) lies in the UV-B or UV-Cregion.
 12. The device of claim 10 or 11 , characterized in that theintegrated UV radiation flux, in particular the Uv-C radiation flux, ofthe low pressure gas discharge lamp (7) above a wavelength of 190 nm, inparticular above 240 nm, is in excess of 50%, preferentially more that75%, of its UV radiation flux, in particular of its UV-C radiation flux.13. The device of one of the preceding claims, characterized in that itcomprises a plurality, in particular more than four and preferentiallymore than eight, of low pressure gas discharge lamps (7).
 14. The deviceof claim 13 , characterized in that it comprises low pressure gasdischarge lamps (7) having mutually differing emission spectra.
 15. Thedevice of one of the preceding claims, characterized in that theelectrical power consumption of the low pressure gas discharge lamps (7)is between 0.2 and 2.5, preferentially between 0.5 and 1.0 watts percentimeter of wavelength.
 16. The device of one of the preceding claims,characterized in that the homogeneity of the UV radiation intensity onthe printed material (9), in particular the UV-B and/or the UV-Cintensity, in the region effective for curing the printing ink (14) issufficiently high that the irradiation intensity deviates from anaverage value by less than 30%, preferentially by less that 20%.
 17. Thedevice of one of the preceding claims, characterized in that itcomprises a plurality of mutually adjacent low pressure gas dischargelamps (7), wherein the separation between the low pressure gas dischargelamps (7) is not more then 30%, preferentially is not more than 20%, ofthe diameter of the low pressure gas discharge lamp (7) bulbs (16). 18.The device of one of the preceding claims, characterized in that itcomprises a plurality of U-shaped low pressure gas discharge lamps (7)disposed in mutual adjacency at the parallel lengthwise sides of theU-shape, wherein the low pressure gas discharge lamps (7) are disposedin alternating opposite directions.
 19. The device of one of thepreceding claims, characterized in that the separation between the lowpressure gas discharge lamps (7) and the printed material (9) is lessthan 5 cm and/or more than 1 cm.
 20. The device of one of the precedingclaims, characterized in that the low pressure gas discharge lamp (7) isa mercury vapor lamp or an amalgam lamp.
 21. The device of one of thepreceding claims, characterized in that it comprises a reflector (5) toreflect the UV light emitted from the low pressure gas discharge lamps(7) onto the curing printing ink (14).
 22. The device of claim 21 ,characterized in that the reflector (5) is stationary.
 23. The device ofclaim 21 or 22 , characterized in that the reflector (5) comprises adielectric mirrored layer and/or a reflecting layer made from anoptically diffuse reflecting material (1).
 24. The device of claim 23 ,characterized in that the optically diffuse reflecting material (1)comprises a matrix of transparent matrix material consisting essentiallyof a curable silicone rubber in which diffuse reflecting particles areimbedded.